The Richat Structure, also known as the Eye of the Sahara and Guelb er Richat, is a prominent circular feature in the Sahara desert of west–central Mauritania near Ouadane. This structure is a deeply eroded, slightly elliptical, 40-km in diameter, dome. The sedimentary rock exposed in this dome range in age from Late Proterozoic within the center of the dome to Ordovician sandstone around its edges. The sedimentary rocks comprising this structure dip outward at 10°-20°. Differential erosion of resistant layers of quartzite has created high-relief circular cuestas. Its center consists of a siliceous breccia covering an area that is at least 3 km in diameter.

Initially interpreted as an asteroid impact structure because of its high degree of circularity, it is now argued to be a highly symmetrical and deeply eroded geologic dome. Despite extensive field and laboratory studies, geologists have found a lack of any credible evidence for shock metamorphism or any type of deformation indicative of a hypervelocity extraterrestrial impact.

As the result of the further analysis of rock samples from this structure, it was concluded that barite had been misidentified as coesite. In addition, the Richat structure lacks the annular depression that characterize large extraterrestrial impact structures of this size. Also, it is quite different from large extraterrestrial impact structures in that the sedimentary strata comprising this structure is remarkably intact and "orderly" and lacking in overturned, steeply dipping strata or disoriented blocks. A more recent multianalytical study on the Richat megabreccias concluded that carbonates within the silica-rich megabreccias were created by low-temperature hydrothermal waters, and that the structure requires special protection and further investigation of its origin.